Initiation of protein synthesis in a cell-free system prepared from rat hepatocytes

Stress-induced perturbations in intracellular amino acids reprogram mRNA translation in osmoadaptation independently of the ISR

The integrated stress response (ISR) plays a pivotal role in adaptation of translation machinery to cellular stress. Here, we demonstrate an ISR-independent osmoadaptation mechanism involving reprogramming of translation via coordinated but independent actions of mTOR and plasma membrane amino acid transporter SNAT2. This biphasic response entails reduced global protein synthesis and mTOR signaling followed by translation of SNAT2. Induction of SNAT2 leads to accumulation of amino acids and reactivation of mTOR and global protein synthesis, paralleled by partial reversal of the early-phase, stress-induced translatome. We propose SNAT2 functions as a molecular switch between inhibition of protein synthesis and establishment of an osmoadaptive translation program involving the formation of cytoplasmic condensates of SNAT2-regulated RNA-binding proteins DDX3X and FUS. In summary, we define key roles of SNAT2 in osmotolerance.

Fluorescence Intensity-Based eIF2B's Guanine Nucleotide-Exchange Factor Activity Assay

Guanine nucleotide-exchange factors (GEFs) activate the function of guanine nucleotide-binding proteins (G-proteins) by promoting the exchange of GDP for GTP on the latter. Here, we describe a protocol for in vitro measurements of the GEF activity of eukaryotic translation initiation factor 2B, eIF2B, toward its substrate eIF2. This protocol provides a relatively simple method for determining the eIF2B's GEF activity in crude cell extracts. The eIF2 heterotrimeric substrate, with phosphorylated or unphosphorylated eIF2α, is prepared by immunoprecipitation, following subsequent loading of a fluorescent BODIPY-FL dye-attached GDP. The exchange of the bound fluorescent GDP molecule for an unlabeled one on eIF2 promoted by eIF2B is monitored kinetically using a fluorescence microplate reader.

Adaptive translational pausing is a hallmark of the cellular response to severe environmental stress

To survive, mammalian cells must adapt to environmental challenges. While the cellular response to mild stress has been widely studied, how cells respond to severe stress remains unclear. We show here that under severe hyperosmotic stress, cells enter a transient hibernation-like state in anticipation of recovery. We demonstrate this adaptive pausing response (APR) is a coordinated cellular response that limits ATP supply and consumption through mitochondrial fragmentation and widespread pausing of mRNA translation. This pausing is accomplished by ribosome stalling at translation initiation codons, which keeps mRNAs poised to resume translation upon recovery. We further show that recovery from severe stress involves ISR (integrated stress response) signaling that permits cell cycle progression, resumption of growth, and reversal of mitochondria fragmentation. Our findings indicate that cells can respond to severe stress via a hibernation-like mechanism that preserves vital elements of cellular function under harsh environmental conditions.

Dietary Protein and Amino Acid Deficiency Inhibit Pancreatic Digestive Enzyme mRNA Translation by Multiple Mechanisms

BACKGROUND & AIMS

Chronic amino acid (AA) deficiency, as in kwashiorkor, reduces the size of the pancreas through an effect on mammalian target of rapamycin complex 1 (mTORC1). Because of the physiological importance of AAs and their role as a substrate, a stimulant of mTORC1, and protein synthesis, we studied the effect of acute protein and AA deficiency on the response to feeding.

METHODS

ICR/CD-1 mice were fasted overnight and refed for 2 hours with 4 different isocaloric diets: control (20% Prot); Protein-free (0% Prot); control (AA-based diet), and a leucine-free (No Leu). Protein synthesis, polysomal profiling, and the activation of several protein translation factors were analyzed in pancreas samples.

RESULTS

All diets stimulated the Protein Kinase-B (Akt)/mTORC1 pathway, increasing the phosphorylation of the kinase Akt, the ribosomal protein S6 (S6) and the formation of the eukaryotic initiation factor 4F (eIF4F) complex. Total protein synthesis and polysome formation were inhibited in the 0% Prot and No Leu groups to a similar extent, compared with the 20% Prot group. The 0% Prot diet partially reduced the Akt/mTORC1 pathway and the activity of the guanine nucleotide exchange factor eIF2B, without affecting eIF2α phosphorylation. The No Leu diet increased the phosphorylation of eIF2α and general control nonderepressible 2, and also inhibited eIF2B activity, without affecting mTORC1. Essential and nonessential AA levels in plasma and pancreas indicated a complex regulation of their cellular transport mechanisms and their specific effect on the synthesis of digestive enzymes.

CONCLUSIONS

These studies show that dietary AAs are important regulators of postprandial digestive enzyme synthesis, and their deficiency could induce pancreatic insufficiency and malnutrition.

A Unique ISR Program Determines Cellular Responses to Chronic Stress

The integrated stress response (ISR) is a homeostatic mechanism induced by endoplasmic reticulum (ER) stress. In acute/transient ER stress, decreased global protein synthesis and increased uORF mRNA translation are followed by normalization of protein synthesis. Here, we report a dramatically different response during chronic ER stress. This chronic ISR program is characterized by persistently elevated uORF mRNA translation and concurrent gene expression reprogramming, which permits simultaneous stress sensing and proteostasis. The program includes PERK-dependent switching to an eIF3-dependent translation initiation mechanism, resulting in partial, but not complete, translational recovery, which, together with transcriptional reprogramming, selectively bolsters expression of proteins with ER functions. Coordination of transcriptional and translational reprogramming prevents ER dysfunction and inhibits "foamy cell" development, thus establishing a molecular basis for understanding human diseases associated with ER dysfunction.

Dietary Methionine Restriction Regulates Liver Protein Synthesis and Gene Expression Independently of Eukaryotic Initiation Factor 2 Phosphorylation in Mice

Background: The phosphorylation of eukaryotic initiation factor 2 (p-eIF2) during dietary amino acid insufficiency reduces protein synthesis and alters gene expression via the integrated stress response (ISR).Objective: We explored whether a Met-restricted (MR) diet activates the ISR to reduce body fat and regulate protein balance.Methods: Male and female mice aged 3-6 mo with either whole-body deletion of general control nonderepressible 2 (Gcn2) or liver-specific deletion of protein kinase R-like endoplasmic reticulum kinase (Perk) alongside wild-type or floxed control mice were fed an obesogenic diet sufficient in Met (0.86%) or an MR (0.12% Met) diet for ≤5 wk. Ala enrichment with deuterium was measured to calculate protein synthesis rates. The guanine nucleotide exchange factor activity of eIF2B was measured alongside p-eIF2 and hepatic mRNA expression levels at 2 d and 5 wk. Metabolic phenotyping was conducted at 4 wk, and body composition was measured throughout. Results were evaluated with the use of ANOVA (P < 0.05).Results: Feeding an MR diet for 2 d did not increase hepatic p-eIF2 or reduce eIF2B activity in wild-type or Gcn2-/- mice, yet many genes transcriptionally regulated by the ISR were altered in both strains in the same direction and amplitude. Feeding an MR diet for 5 wk increased p-eIF2 and reduced eIF2B activity in wild-type but not Gcn2-/- mice, yet ISR-regulated genes altered in both strains similarly. Furthermore, the MR diet reduced mixed and cytosolic but not mitochondrial protein synthesis in both the liver and skeletal muscle regardless of Gcn2 status. Despite the similarities between strains, the MR diet did not increase energy expenditure or reduce body fat in Gcn2-/- mice. Finally, feeding the MR diet to mice with Perk deleted in the liver increased hepatic p-eIF2 and altered body composition similar to floxed controls.Conclusions: Hepatic activation of the ISR resulting from an MR diet does not require p-eIF2. Gcn2 status influences body fat loss but not protein balance when Met is restricted.

IRE1α-Dependent Decay of CReP/Ppp1r15b mRNA Increases Eukaryotic Initiation Factor 2α Phosphorylation and Suppresses Protein Synthesis

The unfolded protein response (UPR) regulates endoplasmic reticulum (ER) homeostasis and protects cells from ER stress. IRE1α is a central regulator of the UPR that activates the transcription factor XBP1s through an unconventional splicing mechanism using its endoribonuclease activity. IRE1α also cleaves certain mRNAs containing XBP1-like secondary structures to promote the degradation of these mRNAs, a process known as regulated IRE1α-dependent decay (RIDD). We show here that the mRNA of CReP/Ppp1r15b, a regulatory subunit of eukaryotic translation initiation factor 2α (eIF2α) phosphatase, is a RIDD substrate. eIF2α plays a central role in the integrated stress response by mediating the translational attenuation to decrease the stress level in the cell. CReP expression was markedly suppressed in XBP1-deficient mice livers due to hyperactivated IRE1α. Decreased CReP expression caused the induction of eIF2α phosphorylation and the attenuation of protein synthesis in XBP1-deficient livers. ER stress also suppressed CReP expression in an IRE1α-dependent manner, which increased eIF2α phosphorylation and consequently attenuated protein synthesis. Taken together, the results of our study reveal a novel function of IRE1α in the regulation of eIF2α phosphorylation and the translational control.

Stress responses. Mutations in a translation initiation factor identify the target of a memory-enhancing compound

The integrated stress response (ISR) modulates messenger RNA translation to regulate the mammalian unfolded protein response (UPR), immunity, and memory formation. A chemical ISR inhibitor, ISRIB, enhances cognitive function and modulates the UPR in vivo. To explore mechanisms involved in ISRIB action, we screened cultured mammalian cells for somatic mutations that reversed its effect on the ISR. Clustered missense mutations were found at the amino-terminal portion of the delta subunit of guanine nucleotide exchange factor (GEF) eIF2B. When reintroduced by CRISPR-Cas9 gene editing of wild-type cells, these mutations reversed both ISRIB-mediated inhibition of the ISR and its stimulatory effect on eIF2B GEF activity toward its substrate, the translation initiation factor eIF2, in vitro. Thus, ISRIB targets an interaction between eIF2 and eIF2B that lies at the core of the ISR.

Isoleucine, leucine, methionine, and threonine effects on mammalian target of rapamycin signaling in mammary tissue

Improved representation of postabsorptive N metabolism in lactating dairy cows requires a better understanding of protein synthesis regulation in the mammary glands. This study aimed to determine the quantitative effects of Ile, Leu, Met, and Thr on the phosphorylation state of signaling proteins that regulate protein synthesis. The experiment used a composite design with a central point, 2 axial points per AA, and a complete 2(4) factorial. All of the other AA were provided at the concentrations in Dulbecco's modified Eagle's medium. The experiment was replicated with tissues from 5 lactating cows. Mammary tissue slices (0.12 ± 0.02 g) were incubated for 4h. Total and site-specific phosphorylated mammalian target of rapamycin (mTOR; Ser2448), eukaryotic elongation factor (eEF) 2 (Thr56), ribosomal protein S6 (Ser235/236), and eukaryotic initiation factor 2α (Ser51) were determined by western immunoblotting. Tissue concentrations of the 4 AA studied responded linearly to media supply. Addition of Ile, Leu, Met, or Thr had no effect on eukaryotic initiation factor 2α phosphorylation. Isoleucine and Thr positively affected mTOR phosphorylation. However, the 2 AA had an antagonistic relationship. Similarly, Ile linearly increased ribosomal protein S6 phosphorylation, and Thr inhibited the Ile effect. In addition, eEF2 phosphorylation was linearly decreased by Ile and Leu. Threonine curvilinearly decreased eEF2 phosphorylation, Ile and Leu negatively interacted on eEF2, and Thr tended to inhibit Leu effects on eEF2. This work demonstrated saturable responses and interactions between AA on activation of the mTOR pathway. Incorporation of these concepts into milk protein response models will help to improve milk and milk protein yield predictions and increase postabsorptive N efficiency and reduce N excretion by dairy cows.

Toll-like receptor activation suppresses ER stress factor CHOP and translation inhibition through activation of eIF2B

Activation of toll-like receptors (TLRs) induces the endoplasmic reticulum (ER) Unfolded Protein Response (UPR) to accommodate essential protein translation^1,2. However, despite increased p-eIF2α, a TLR-TRIF-dependent pathway assures that the cells avoid CHOP induction, apoptosis, and translational suppression of critical proteins³. Because p-eIF2α decreases the functional interaction of eIF2 with eIF2B, a guanine nucleotide exchange factor (GEF), we explored the hypothesis that TLR-TRIF signaling activates eIF2B-GEF activity to counteract the effects of p-eIF2α. We now show that TLR-TRIF signaling activates eIF2B-GEF through PP2A-mediated Ser-dephosphorylation of the eIF2B ε-subunit. PP2A itself is activated by decreased Src-family-kinase-induced Tyr-phosphorylation of its catalytic subunit. Each of these processes are required for TLR-TRIF-mediated CHOP suppression in ER-stressed cells in vitro and in vivo. Thus, in the setting of prolonged, physiologic ER stress, a unique TLR-TRIF-dependent translational control pathway enables cells to carry out essential protein synthesis and avoid CHOP-induced apoptosis while still benefitting from the protective arms of the UPR.

A mouse model for eukaryotic translation initiation factor 2B-leucodystrophy reveals abnormal development of brain white matter

Eukaryotic translation initiation factor 2B is a major housekeeping complex that governs the rate of global protein synthesis under normal and stress conditions. Mutations in any of its five subunits lead to leucoencephalopathy with vanishing white matter, an inherited chronic-progressive fatal brain disease with unknown aetiology, which is among the most prevalent childhood white matter disorders. We generated the first animal model for the disease by introducing a point mutation into the mouse Eif2b5 gene locus, leading to R132H replacement corresponding to the clinically significant human R136H mutation in the catalytic subunit. In contrast to human patients, mice homozygous for the mutant Eif2b5 allele (Eif2b5(R132H/R132H) mice) enable multiple analyses under a defined genetic background during the pre-symptomatic stages and during recovery from a defined brain insult. Time-course magnetic resonance imaging revealed for the first time the delayed development of the brain white matter due to the mutation. Electron microscopy demonstrated a higher proportion of small-calibre nerve fibres. Immunohistochemistry detected an abnormal abundance of oligodendrocytes and astrocytes in the brain of younger animals, as well as an abnormal level of major myelin proteins. Most importantly, mutant mice failed to recover from cuprizone-induced demyelination, reflecting an increased sensitivity to brain insults. The anomalous development of white matter in Eif2b5(R132H/R132H) mice underscores the importance of tight translational control to normal myelin formation and maintenance.

Activation of the mammalian target of rapamycin complex 1 is both necessary and sufficient to stimulate eukaryotic initiation factor 2Bvarepsilon mRNA translation and protein synthesis

In a previous study we demonstrated a requirement for activation of mTORC1 in the stimulation of eIF2Bepsilon mRNA translation in skeletal muscle in response to resistance exercise. Although that study established the necessity of mTORC1 activation, the experimental model used did not lend itself readily to address the question of whether or not mTORC1 activation was sufficient to produce the response. Therefore, the present study was designed to address the sufficiency of mTORC1 activation, using cultures of Rat2 fibroblasts in which mTORC1 signaling was repressed by serum/leucine-depletion and stimulated by repletion of leucine and/or IGF-1. Repletion with leucine and IGF-1 caused a shift of eIF2Bepsilon mRNA into actively translating polysomes and a stimulation of new eIF2Bepsilon protein synthesis, but had no effect on mRNAs encoding the other four eIF2B subunits. Stimulation of eIF2Bepsilon translation was reversed by pre-treatment with the mTORC1 inhibitor rapamycin. Exogenous overexpression of FLAG-Rheb, a proximal activator of mTORC1, also caused a re-distribution of eIF2Bepsilon mRNA into polysomes and a stimulation of eIF2Bepsilon protein synthesis. The stimulation of eIF2Bepsilon mRNA translation occurred in the absence of any effect on eIF2Bepsilon mRNA abundance. RNAi-mediated knockdown of eIF2Bepsilon resulted in reduced cellular proliferation, a result that phenocopied the known cytostatic effect of mTORC1 repression. Overall the results demonstrate that activation of mTORC1 is both necessary and sufficient to stimulate eIF2Bepsilon mRNA translation and that this response may represent a novel mechanism through which mTORC1 can affect mRNA translation initiation, rates of protein synthesis, and cellular growth/proliferation.

Kinectin-dependent assembly of translation elongation factor-1 complex on endoplasmic reticulum regulates protein synthesis

Kinectin is an integral membrane protein with many isoforms primarily found on the endoplasmic reticulum. It has been found to bind kinesin, Rho GTPase, and translation elongation factor-1delta. None of the existing models for the quaternary organization of the elongation factor-1 complex in higher eukaryotes involves kinectin. We have investigated here the assembly of the elongation factor-1 complex onto endoplasmic reticulum via kinectin using in vitro and in vivo assays. We established that the entire elongation factor-1 complex can be anchored to endoplasmic reticulum via kinectin, and the interacting partners are as follows. Kinectin binds EF-1delta, which in turn binds EF-1gamma but not EF-1beta; EF-1gamma binds EF-1delta and EF-1beta but not kinectin. In vivo splice blocking of the kinectin exons 36 and 37 produced kinectin lacking the EF-1delta binding domain, which disrupted the membrane localization of EF-1delta, EF-1gamma, and EF-1beta on endoplasmic reticulum, similar to the disruptions seen with the overexpression of kinectin fragments containing the EF-1delta binding domain. The disruptions of the EF-1delta/kinectin interaction inhibited expression of membrane proteins but enhanced synthesis of cytosolic proteins in vivo. These findings suggest that anchoring the elongation factor-1 complex onto endoplasmic reticulum via EF-1delta/kinectin interaction is important for regulating protein synthesis in eukaryotic cells.

Heightened stress response in primary fibroblasts expressing mutant eIF2B genes from CACH/VWM leukodystrophy patients

Childhood ataxia with central nervous system hypomyelination (CACH), also called vanishing white matter (VWM) leukoencephalopathy, is a fatal genetic disease caused by mutations in eukaryotic initiation factor 2B (eIF2B) genes. The five subunits eIF2B factor is critical for translation initiation under normal conditions and regulates protein synthesis in response to cellular stresses. Primary fibroblasts from CACH/VWM patients and normal individuals were used to measure basal eIF2B activity as well as global protein synthesis and ATF4 induction in response to stress in the endoplasmic reticulum. We show that although the cells expressing mutant eIF2B genes respond normally to stress conditions by reduced global translation rates, they exhibit significantly greater increase in ATF4 induction compared to normal controls despite equal levels of stress and activity of the upstream eIF2alpha kinase. This heightened stress response observed in primary fibroblasts that suffer from minor loss of basal eIF2B activity may be employed as an initial screening tool for CACH/VWM leukodystrophy.

Effect of leptin on liver alcohol dehydrogenase

The effect of leptin on liver alcohol dehydrogenase (ADH) was determined in male rats. Administration of one or three daily doses of leptin (1microg/g of body weight intraperitoneally) increased ADH activity. Leptin enhanced ADH synthesis without an effect on ADH degradation. Leptin did not change ADH mRNA, indicating that the effect of leptin in enhancing ADH occurs at the post-transcriptional level. Leptin increased eukaryotic initiation factor (eIF) 2alpha, eIF2B activity, and the eIF4E-eIF4G complex, while it decreased the inhibitory complex of eIF4E with the eIF4E-binding protein-1 (4E-BP1). Leptin increased mammalian target of rapamycin (mTor) that phosphorylates 4E-BP1. In conclusion, leptin increases liver ADH activity and ADH protein due to an increase in synthesis which occurs at the post-transcriptional level. The effect of leptin in enhancing translational initiating factors may be of significance in the regulation not only of ADH but also of many other proteins.

Decreased guanine nucleotide exchange factor activity in eIF2B-mutated patients

Mutations in each of the five eucaryotic initiation factor 2B (eIF2B) subunits have been found in leukodystrophies of various severity: Cree leukoencephalopathy, childhood ataxia with central hypomyelination/leukodystrophy with vanishing white matter and ovarioleukodystrophy. A continuum was observed from fatal infantile forms to adult forms without neurological deterioration. Disease severity was found to correlate with the age at disease onset and the specific amino-acid substitution. In order to analyze the functional consequences of eIF2B mutations, we measured the guanine nucleotide exchange factor (GEF) activity of eIF2B in transformed lymphocytes from 30 affected patients carrying mutations in eIF2B compared to 10 unaffected heterozygotes and 22 controls without eIF2B mutations. A significant decrease of 20-70% in GEF activity was observed in all mutated cells. The severity of this decrement of GEF activity correlated with age at onset of the disease. These results suggest that a deficiency in GEF activity underlies the encephalopathy associated eIF2B-related disease. Our study demonstrates that the evaluation of the GEF activity in transformed lymphocytes represents an interesting alternative test to the systematic screening of the five EIF2B genes. This relevant cellular model may also be used to test the functional impact of different molecules on the GEF activity for future therapeutic strategies.

Feeding activates protein synthesis in mouse pancreas at the translational level without increase in mRNA

To determine the mechanism of meal-regulated synthesis of pancreatic digestive enzymes, we studied the effect of fasting and refeeding on pancreatic protein synthesis, relative mRNA levels of digestive enzymes, and activation of the translational machinery. With the use of the flooding dose technique with L-[3H]phenylalanine, morning protein synthesis in the pancreas of Institute for Cancer Research mice fed ad libitum was 7.9 +/- 0.3 nmol phenylalanine.10 min(-1).mg protein(-1). Prior fasting for 18 h reduced total protein synthesis to 70 +/- 1.4% of this value. Refeeding for 2 h, during which the mice consumed 29% of their daily food intake, increased protein synthesis to 117.3 +/- 4.9% of the control level. Pancreatic mRNA levels of amylase, lipases, trypsins, chymotrypsin, elastases, as well as those for several housekeeping genes tested were not significantly changed after refeeding compared with fasted mice. By contrast, the major translational control pathway involving Akt, mTOR, and S6K was strongly regulated by fasting and refeeding. Fasting for 18 h decreased phosphorylation of ribosomal protein S6 to almost undetectable levels, and refeeding highly increased it. The most highly phosphorylated form of the eIF4E binding protein (4E-BP1) made up the 14.6% of total 4E-BP1 in normally fed animals, was only 2.8% after fasting, and was increased to 21.4% after refeeding. This was correlated with an increase in the formation of the eIF4E-eIF4G complex after refeeding. By contrast, feeding did not affect eIF2B activity. Thus food intake stimulates pancreatic protein synthesis and translational effectors without increasing digestive enzyme mRNA levels.

Glucocorticoids modulate amino acid-induced translation initiation in human skeletal muscle

Amino acids are unique anabolic agents in that they nutritively signal to mRNA translation initiation and serve as substrates for protein synthesis in skeletal muscle. Glucocorticoid excess antagonizes the anabolic action of amino acids on protein synthesis in laboratory animals. To examine whether excessive glucocorticoids modulate mixed amino acid-signaled translation initiation in human skeletal muscle, we infused an amino acid mixture (10% Travasol) systemically to 16 young healthy male volunteers for 6 h in the absence (n = 8) or presence (n = 8) of glucocorticoid excess (dexamethasone 2 mg orally every 6 h for 3 days). Vastus lateralis muscles were biopsied before and after amino acid infusion, and the phosphorylation of eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1), ribosomal protein S6 kinase (p70(S6K)), and eIF2alpha and the guanine nucleotide exchange activity of eIF2B were measured. Systemic infusion of mixed amino acids significantly stimulated the phosphorylation of 4E-BP1 (P < 0.04) and p70(S6K) (P < 0.001) and the dephosphorylation of eIF2alpha (P < 0.003) in the control group. Dexamethasone treatment did not alter the basal phosphorylation state of 4E-BP1, p70(S6K), or eIF2alpha; however, it abrogated the stimulatory effect of amino acid infusion on the phosphorylation of 4E-BP1 (P = 0.31) without affecting amino acid-induced phosphorylation of p70(S6K) (P = 0.002) or dephosphorylation of eIF2alpha (P = 0.003). Neither amino acid nor dexamethasone treatment altered the guanine nucleotide exchange activity of eIF2B. We conclude that changes of amino acid concentrations within the physiological range stimulate mRNA translation by enhancing the binding of mRNA to the 43S preinitiation complex, and the activity of p70(S6K) and glucocorticoid excess blocks the former action in vivo in human skeletal muscle.

Calcineurin is required for translational control of protein synthesis in rat pancreatic acini

CCK increases the rate of net protein synthesis in rat pancreatic acini by activating initiation and elongation factors required for translation. The immunosuppressant FK506 inhibits the Ca2+-calmodulin-dependent phosphatase calcineurin in pancreatic acinar cells and blocks pancreatic growth induced by chronic CCK treatment. To test a requirement for calcineurin in the activation of the translational machinery stimulated by CCK, we evaluated the effects of FK506 on protein synthesis and on regulatory initiation and elongation factors in rat pancreatic acini in vitro. CCK acutely increased protein synthesis in acini from normal rats with a maximum increase at 100 pM CCK to 170 ± 11% of control. The immunosuppressant FK506 dose-dependently inhibited CCK-stimulated protein synthesis over the same concentration range that blocked calcineurin activity, as assessed by dephosphorylation of the calcineurin substrate calcium-regulated heat-stable protein of 24 kDa. Another immunosuppressant, cyclosporin A, inhibited protein synthesis, but its effects appeared more complex. FK506 also inhibited protein synthesis stimulated by bombesin and carbachol. FK506 did not significantly affect the activity of the initiation factor-2B, or the phosphorylation of the initiation factor-2α, ribosomal protein protein S6, or the mRNA cap binding protein eukaryotic initiation factor (eIF) 4E. Instead, blockade of calcineurin with FK506 reduced the phosphorylation of the eIF4E binding protein, reduced the formation of the eIF4F complex, and increased the phosphorylation of eukaryotic elongation factor 2. From these results, we conclude that calcineurin activity is required for protein synthesis, and this action may be related to an effect on the formation of the mRNA cap binding complex and the elongation processes.

Regulation of neonatal liver protein synthesis by insulin and amino acids in pigs

The high efficiency of protein deposition during the neonatal period is driven by high rates of protein synthesis, which are maximally stimulated after feeding. Infusion of amino acids, but not insulin, reproduces the feeding-induced stimulation of liver protein synthesis. To determine whether amino acid-stimulated liver protein synthesis is independent of insulin in neonates, and to examine the role of amino acids and insulin in the regulation of translation initiation in neonatal liver, we performed pancreatic glucose-amino acid clamps in overnight-fasted 7-day-old pigs. Pigs (n = 9-12/group) were infused with insulin at 0, 10, 22, and 110 ng.kg(-0.66).min(-1) to achieve 0, 2, 6, and 30 microU/ml insulin, respectively. At each insulin dose, amino acids were maintained at fasting or fed levels or, in conjunction with the highest insulin dose, allowed to fall to below fasting levels. Insulin had no effect on the fractional rate of protein synthesis in liver. Amino acids increased fractional protein synthesis rates in liver at each dose of insulin, including the 0 microU/ml dose. There was a dose-response effect of amino acids on liver protein synthesis. Amino acids and insulin increased protein S6 kinase and 4E-binding protein 1 (4E-BP1) phosphorylation; however, only amino acids decreased formation of the inactive 4E-BPI.eukaryotic initiation factor-4E (eIF4E) complex. The results suggest that amino acids regulate liver protein synthesis in the neonate by modulating the availability of eIF4E for 48S ribosomal complex formation and that this response does not require insulin.

Caerulein-induced acute pancreatitis inhibits protein synthesis through effects on eIF2B and eIF4F

Acute pancreatitis (AP) has been shown in some studies to inhibit total protein synthesis in the pancreas, whereas in other studies, protein synthesis was not affected. Previous in vitro work has shown that high concentrations of cholecystokinin both inhibit protein synthesis and inhibit the activity of the guanine nucleotide exchange factor eukaryotic initiation factor (eIF)2B by increasing the phosphorylation of eIF2alpha. We therefore evaluated in C57BL/6 mice the effects of caerulein-induced AP on pancreatic protein synthesis, eIF2B activity and other protein translation regulatory mechanisms. Repetitive hourly injections of caerulein were administered at 50 microg/kg ip. Pancreatic protein synthesis was reduced 10 min after the initial caerulein administration and was further inhibited after three and five hourly injections. Caerulein inhibited the two major regulatory points of translation initiation: the activity of the guanine nucleotide exchange factor eIF2B (with an increase of eIF2alpha phosphorylation) and the formation of the eIF4F complex due, in part, to degradation of eIF4G. This inhibition was not accounted for by changes in the upstream stimulatory pathway, because caerulein activated Akt as well as phosphorylating the downstream effectors of mTOR, 4E-BP1, and ribosomal protein S6. Caerulein also decreased the phosphorylation of the eukaryotic elongation factor 2, implying that this translation factor was not inhibited in AP. Thus the inhibition of pancreatic protein synthesis in this model of AP most likely results from the inhibition of translation initiation as a result of increased eIF2alpha phosphorylation, reduction of eIF2B activity, and the inhibition of eIF4F complex formation.

Regulation of translation initiation by insulin and amino acids in skeletal muscle of neonatal pigs

Previous studies have shown that intravenous infusion of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates and that insulin and amino acids act independently to produce this effect. The goal of the present study was to delineate the regulatory roles of insulin and amino acids on muscle protein synthesis in neonates by examining translational control mechanisms, specifically the eukaryotic translation initiation factors (eIFs), which enable coupling of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. Insulin secretion was blocked by somatostatin in fasted 7-day-old pigs (n = 8-12/group), insulin was infused to achieve plasma levels of approximately 0, 2, 6, and 30 microU/ml, and amino acids were clamped at fasting or fed levels or, at the high insulin dose, below fasting. Both insulin and amino acids increased the phosphorylation of ribosomal protein S6 kinase (S6K1) and the eIF4E-binding protein (4E-BP1), decreased the binding of 4E-BP1 to eIF4E, increased eIF4E binding to eIF4G, and increased fractional protein synthesis rates but did not affect eIF2B activity. In the absence of insulin, amino acids had no effect on these translation initiation factors but increased the protein synthesis rates. Raising insulin from below fasting to fasting levels generally did not alter translation initiation factor activity but raised protein synthesis rates. The phosphorylation of S6K1 and 4E-BP1 and the amount of 4E-BP1 bound to eIF4E and eIF4E bound to eIF4G were correlated with insulin level, amino acid level, and protein synthesis rate. Thus insulin and amino acids regulate muscle protein synthesis in skeletal muscle of neonates by modulating the availability of eIF4E for 48S ribosomal complex assembly, although other processes also must be involved.

The GCN2 eIF2alpha kinase is required for adaptation to amino acid deprivation in mice

The GCN2 eIF2alpha kinase is essential for activation of the general amino acid control pathway in yeast when one or more amino acids become limiting for growth. GCN2's function in mammals is unknown, but must differ, since mammals, unlike yeast, can synthesize only half of the standard 20 amino acids. To investigate the function of mammalian GCN2, we have generated a Gcn2(-/-) knockout strain of mice. Gcn2(-/-) mice are viable, fertile, and exhibit no phenotypic abnormalities under standard growth conditions. However, prenatal and neonatal mortalities are significantly increased in Gcn2(-/-) mice whose mothers were reared on leucine-, tryptophan-, or glycine-deficient diets during gestation. Leucine deprivation produced the most pronounced effect, with a 63% reduction in the expected number of viable neonatal mice. Cultured embryonic stem cells derived from Gcn2(-/-) mice failed to show the normal induction of eIF2alpha phosphorylation in cells deprived of leucine. To assess the biochemical effects of the loss of GCN2 in the whole animal, liver perfusion experiments were conducted. Histidine limitation in the presence of histidinol induced a twofold increase in the phosphorylation of eIF2alpha and a concomitant reduction in eIF2B activity in perfused livers from wild-type mice, but no changes in livers from Gcn2(-/-) mice.

Regulation of translation initiation by amino acids in eukaryotic cells

The translation of mRNA in eukaryotic cells is regulated by amino acids through multiple mechanisms. One such mechanism involves activation of mTOR (Fig. 1). mTOR controls a myriad of downstream effectors, including RNA polymerase I, S6K1, 4E-BP1, and eEF2 kinase. In yeast, and probably in higher eukaryotes, mTOR signals through Tap42p/alpha 4 to regulate protein phosphatases. Through phosphorylation of Tap42p/alpha 4, mTOR abrogates dephosphorylation of the downstream effectors by PP2 A and/or PP6, resulting in their increased phosphorylation. Although at this time still speculative, in vitro results using mTOR immunoprecipitates suggest that mTOR, or an associated kinase, may also be directly involved in phosphorylating some effectors. Enhanced RNA polymerase I activity results in increased transcription of rDNA genes, whereas increased S6K1 activity promotes preferential translation of TOP mRNAs, such as those encoding ribosomal proteins. Together, stimulated RNA polymerase I and S6K1 activities enhance ribosome biogenesis, increasing the translational capacity of the cell. Phosphorylation of 4E-BP1 prohibits its association with eIF4E, allowing eIF4E to bind to eIF4G and form the active eIF4F complex. Increased eIF4F formation preferentially stimulates translation of mRNAs containing long, highly-structured 5' UTRs. Finally, amino acids cause inhibition of the eEF2 kinase, resulting in an increase in the proportion of eEF2 in the active, dephosphorylated form. By inhibiting eEF2 phosphorylation, amino acids may not only stimulate translation elongation, but may also prevent activation of GCN2 by enhancing the rate of removal of deacylated tRNA from the P-site on the ribosome; a potential activator of GCN2. GCN2 may also be regulated directly by the accumulation of deacylated-tRNA caused by treatment with inhibitors of tRNA synthetases or in cells incubated in the absence of essential amino acids. However, because the Km of the tRNA synthetases for amino acids is well above the amino acid concentrations found in plasma of fasted animals, such a mechanism may not be operative in mammals in vivo. Activation of GCN2 results in increased phosphorylation of the alpha-subunit of eIF2, which in turn causes inhibition of eIF2B. Thus, by preventing activation of GCN2, amino acids preserve eIF2B activity, which promotes translation of all mRNAs, i.e., global protein synthesis is enhanced.

Effect of CCK and intracellular calcium to regulate eIF2B and protein synthesis in rat pancreatic acinar cells

Pancreatic secretagogues enhance acinar protein synthesis at physiological concentrations and inhibit protein synthesis at high concentrations. We investigated the potential role in this process of the eukaryotic translation initiation factor (eIF)2B. Cholecystokinin (CCK) at 10-100 pM did not significantly affect eIF2B activity, which averaged 35.4 nmol guanosine 5'-diphosphate exchanged per minute per milligram protein under control conditions; higher CCK concentrations reduced eIF2B activity to 38.2% of control. Carbamylcholine chloride (Carbachol, CCh), A-23187, and thapsigargin also inhibited eIF2B and protein synthesis, whereas bombesin and the CCK analog JMV-180 were without effect. Previous studies have shown that eIF2B can be negatively regulated by glycogen synthase kinase-3 (GSK-3). However, GSK-3 activity, as assessed by phosphorylation state, was inhibited at high concentrations of CCK, an effect that should have stimulated, rather than repressed, eIF2B activity. An alternative mechanism for regulating eIF2B is through phosphorylation of the alpha-subunit of eIF2, which converts it into an inhibitor of eIF2B. CCK, CCh, A-23187, and thapsigargin all enhanced eIF2alpha phosphorylation, suggesting that eIF2B activity is regulated by eIF2alpha phosphorylation under these conditions. Removal of Ca(2+) from the medium enhanced the inhibitory action of CCK on both protein synthesis and eIF2B activity as well as further increasing eIF2alpha phosphorylation. Although it is likely that other mechanisms account for the stimulation of acinar protein synthesis, these results suggest that the inhibition of acinar protein synthesis by CCK occurs as a result of depletion of Ca(2+) from the endoplasmic reticulum lumen leading to phosphorylation of eIF2alpha and inhibition of eIF2B.

The double-stranded RNA-activated protein kinase PKR is dispensable for regulation of translation initiation in response to either calcium mobilization from the endoplasmic reticulum or essential amino acid starvation

The alpha-subunit of eukaryotic initiation factor eIF2 is a preferred substrate for the double-stranded RNA-activated protein kinase, PKR. Phosphorylation of eIF2alpha converts the factor from a substrate into a competitive inhibitor of the guanine nucleotide exchange factor, eIF2B, leading to a decline in mRNA translation. Early studies provided evidence implicating PKR as the kinase that phosphorylates eIF2alpha under conditions of cell stress such as the accumulation of misfolded proteins in the lumen of the endoplasmic reticulum, i.e., the unfolded protein response (UPR). However, the recent identification of a trans-microsomal membrane eIF2alpha kinase, termed PEK or PERK, suggests that this kinase, and not PKR, might be the kinase that is activated by misfolded protein accumulation. Similarly, genetic studies in yeast provide compelling evidence that a kinase termed GCN2 phosphorylates eIF2alpha in response to amino acid deprivation. However, no direct evidence showing activation of the mammalian homologue of GCN2 by amino acid deprivation has been reported. In the present study, we find that in fibroblasts treated with agents that promote the UPR, protein synthesis is inhibited as a result of a decrease in eIF2B activity. Furthermore, the reduction in eIF2B activity is associated with enhanced phosphorylation of eIF2alpha. Importantly, the magnitude of the change in each parameter is identical in wildtype cells and in fibroblasts containing a chromosomal deletion in the PKR gene (PKR-KO cells). In a similar manner, we find that during amino acid deprivation the inhibition of protein synthesis and extent of increase in eIF2alpha phosphorylation are identical in wildtype and PKR-KO cells. Overall, the results show that PKR is not required for increased eIF2alpha phosphorylation or inhibition of protein synthesis under conditions promoting the UPR or in response to amino acid deprivation.

Developmental changes in the feeding-induced stimulation of translation initiation in muscle of neonatal pigs

The rapid gain in skeletal muscle mass in the neonate is associated with a marked elevation in skeletal muscle protein synthesis in response to feeding. The feeding-induced response decreases with development. To determine whether the response to feeding is regulated at the level of translation initiation, the expression, phosphorylation, and function of a number of eukaryotic initiation factors (eIF) were examined. Pigs at 7 and 26 days of age were either fasted overnight or fed porcine milk after an overnight fast. In muscle of 7-day-old pigs, the hyperphosphorylated form of the eIF4E repressor protein, 4E-binding protein 1 (4E-BP1), was undetectable in the fasting state but rose to 60% of total 4E-BP1 after feeding; eIF4E phosphorylation was unaffected by feeding status. The amount of eIF4E in the inactive 4E-BP1. eIF4E complex was reduced by 80%, and the amount of eIF4E in the active eIF4E. eIF4G complex was increased 14-fold in muscle of 7-day-old pigs after feeding. The amount of 70-kDa ribosomal protein S6 (p70(S6)) kinase in the hyperphosphorylated form rose 2.5-fold in muscle of 7-day-old pigs after feeding. Each of these feeding-induced responses was blunted in muscle of 26-day-old pigs. eIF2B activity in muscle was unaffected by feeding status but decreased with development. Feeding produced similar changes in eIF characteristics in liver and muscle; however, the developmental changes in liver were not as apparent as in skeletal muscle. Thus the results demonstrate that the developmental change in the acute stimulation of skeletal muscle protein synthesis by feeding is regulated by the availability of eIF4E for 48S ribosomal complex formation. The results further suggest that the overall developmental decline in skeletal muscle protein synthesis involves regulation by eIF2B.

Signal transduction pathways that contribute to increased protein synthesis during T-cell activation

Protein synthesis rates were maximally stimulated in human lymphocytes by ionomycin and the phorbol ester PMA (I+P), which promotes proliferation, whereas PMA alone, which does not promote proliferation, stimulated protein synthesis to a lesser degree. Three translation-associated activities, eIF4E phosphorylation, eIF2B activity and 4E-BP1 phosphorylation also increased with stimulation by I+P and PMA, but only 4E-BP1 phosphorylation was differentially stimulated by these conditions. Correspondingly, signaling pathways activated in T cells were probed for their connection to these activities. Immunosuppressants FK506 and rapamycin partially blocked the protein synthesis rate increases by I+P stimulation. FK506 had less of an inhibitory effect with PMA stimulation suggesting that its mechanism mostly affected ionomycin-activated signals. I+P and PMA equally stimulated phosphorylation of ERK1/2, but I+P more strongly stimulated Akt, and p70(S6K) phosphorylation. An inhibitor that blocks ERK1/2 phosphorylation only slightly reduced protein synthesis rates stimulated by I+P or PMA, but greatly reduced eIF4E phosphorylation and eIF2B activity. In contrast, inhibitors of the PI-3 kinase and mTOR pathways strongly blocked early protein synthesis rate stimulated by I+P and PMA and also blocked 4E-BP1 phosphorylation and release of eIF4E suggesting that these pathways regulate protein synthesis activities, which are important for proliferation in T cells.

Identification of domains within the epsilon-subunit of the translation initiation factor eIF2B that are necessary for guanine nucleotide exchange activity and eIF2B holoprotein formation

Eukaryotic translation initiation factor, eIF2B, is a guanine nucleotide exchange factor (GEF) composed of five dissimilar subunits. eIF2B is important for regenerating GTP-bound eIF2 during the initiation process. This event is obligatory for eIF2 to bind initiator methionyl-tRNA, forming the ternary initiation complex. In the current investigation, deletion mutants of the catalytic subunit, eIF2B epsilon, were constructed to identify regions that are necessary for eIF2B catalytic activity and formation of the holoprotein. We used the baculovirus expression system to coexpress wild-type and truncated forms of the epsilon-subunit of mammalian eIF2B (eIF2B epsilon) with the other four subunits (alpha, beta, gamma, delta) of the protein in Sf9 cells. Removal of either the N- or the C-terminal conserved domains of eIF2B epsilon resulted in a significant loss of GEF activity and reduced or abolished interaction with the alpha-, gamma- and delta-subunits of eIF2B. Removal of the C-terminal 552 amino acids of eIF2B epsilon markedly reduced its interaction with the beta-subunit of eIF2 whereas loss of the N-terminal 431 amino acids did not. The results suggest that intact eIF2B epsilon is required for full catalytic activity and formation of the eIF2B holoprotein. In contrast, the C-terminal domain of eIF2B epsilon is sufficient alone for binding the beta-subunit of its substrate, eIF2, in vitro.

Leucine, glutamine, and tyrosine reciprocally modulate the translation initiation factors eIF4F and eIF2B in perfused rat liver

Leucine, glutamine, and tyrosine, three amino acids playing key modulatory roles in hepatic proteolysis, were evaluated for activation of signaling pathways involved in regulation of liver protein synthesis. Furthermore, because leucine signals to effectors that lie distal to the mammalian target of rapamycin, these downstream factors were selected for study as candidate mediators of amino acid signaling. Using the perfused rat liver as a model system, we observed a 25% stimulation of protein synthesis in response to balanced hyperaminoacidemia, whereas amino acid imbalance due to elevated concentrations of leucine, glutamine, and tyrosine resulted in a protein synthetic depression of roughly 50% compared with normoaminoacidemic controls. The reduction in protein synthesis accompanying amino acid imbalance became manifest at high physiologic concentrations and was dictated by the guanine nucleotide exchange activity of translation initiation factor eIF2B. Paradoxically, this phenomenon occurred concomitantly with assembly of the mRNA cap recognition complex, eIF4F as well as activation of the 70-kDa ribosomal S6 kinase, p70(S6k). Dual and reciprocal modulation of eIF4F and eIF2B was leucine-specific because isoleucine, a structural analog, was ineffective in these regards. Thus, we conclude that amino acid imbalance, heralded by leucine, initiates a liver-specific translational fail-safe mechanism that deters protein synthesis under unfavorable circumstances despite promotion of the eIF4F complex.

Implication of eIF2B rather than eIF4E in the regulation of global protein synthesis by amino acids in L6 myoblasts

The present study was designed to investigate the mechanism through which leucine and histidine regulate translation initiation in L6 myoblasts. The results show that both amino acids stimulate initiation and coordinately regulate the activity of eukaryotic initiation factor eIF2B. The changes in eIF2B activity could be explained in part by modulation of the phosphorylation state of the alpha-subunit of eIF2. The activity changes might also be a result of modulation of the phosphorylation state of the eIF2B epsilon-subunit, because deprivation of either amino acid caused a decrease in eIF2Bepsilon kinase activity. Leucine, but not histidine, additionally caused a redistribution of eIF4E from the inactive eIF4E.4E-BP1 complex to the active eIF4E.eIF4G complex. The redistribution was a result of increased phosphorylation of 4E-BP1. The changes in 4E-BP1 phosphorylation and eIF4E redistribution associated with leucine deprivation were not observed in the presence of insulin. However, the leucine- and histidine-induced alterations in global protein synthesis and eIF2B activity were maintained in the presence of the hormone. Overall, the results suggest that both leucine and histidine regulate global protein synthesis through modulation of eIF2B activity. Furthermore, under the conditions employed herein, alterations in eIF4E availability are not rate-controlling for global protein synthesis but might be necessary for regulation of translation of specific mRNAs.

Isolation of the gene encoding the Drosophila melanogaster homolog of the Saccharomyces cerevisiae GCN2 eIF-2alpha kinase

Genomic and cDNA clones homologous to the yeast GCN2 eIF-2alpha kinase (yGCN2) were isolated from Drosophila melanogaster. The identity of the Drosophila GCN2 (dGCN2) gene is supported by the unique combination of sequence encoding a protein kinase catalytic domain and a domain homologous to histidyl-tRNA synthetase and by the ability of dGCN2 to complement a deletion mutant of the yeast GCN2 gene. Complementation of Deltagcn2 in yeast by dGCN2 depends on the presence of the critical regulatory phosphorylation site (serine 51) of eIF-2alpha. dGCN2 is composed of 10 exons encoding a protein of 1589 amino acids. dGCN2 mRNA is expressed throughout Drosophila development and is particularly abundant at the earliest stages of embryogenesis. The dGCN2 gene was cytogenetically and physically mapped to the right arm of the third chromosome at 100C3 in STS Dm2514. The discovery of GCN2 in higher eukaryotes is somewhat unexpected given the marked differences between the amino acid biosynthetic pathways of yeast vs. Drosophila and other higher eukaryotes. Despite these differences, the presence of GCN2 in Drosophila suggests at least partial conservation from yeast to multicellular organisms of the mechanisms responding to amino acid deprivation.

Regulation of guanine nucleotide exchange through phosphorylation of eukaryotic initiation factor eIF2alpha. Role of the alpha- and delta-subunits of eiF2b

The guanine nucleotide exchange activity of eIF2B plays a key regulatory role in the translation initiation phase of protein synthesis. The activity is markedly inhibited when the substrate, i. e. eIF2, is phosphorylated on Ser51 of its alpha-subunit. Genetic studies in yeast implicate the alpha-, beta-, and delta-subunits of eIF2B in mediating the inhibition by substrate phosphorylation. However, the mechanism involved in the inhibition has not been defined biochemically. In the present study, we have coexpressed the five subunits of rat eIF2B in Sf9 cells using the baculovirus system and have purified the recombinant holoprotein to >90% homogeneity. We have also expressed and purified a four-subunit eIF2B complex lacking the alpha-subunit. Both the five- and four-subunit forms of eIF2B exhibit similar rates of guanine nucleotide exchange activity using unphosphorylated eIF2 as substrate. The five-subunit form is inhibited by preincubation with phosphorylated eIF2 (eIF2(alphaP)) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. In contrast, eIF2B lacking the alpha-subunit is insensitive to inhibition by eIF2(alphaP) and is able to exchange guanine nucleotide using eIF2(alphaP) as substrate at a faster rate compared with five-subunit eIF2B. Finally, a double point mutation in the delta-subunit of eIF2B has been identified that results in insensitivity to inhibition by eIF2(alphaP) and exhibits little exchange activity when eIF2(alphaP) is used as substrate. The results provide the first direct biochemical evidence that the alpha- and delta-subunits of eIF2B are involved in mediating the effect of substrate phosphorylation.

Disruption of cellular translational control by a viral truncated eukaryotic translation initiation factor 2alpha kinase homolog

Phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha) is a common cellular mechanism to limit protein synthesis in stress conditions. Baculovirus PK2, which resembles the C-terminal half of a protein kinase domain, was found to inhibit both human and yeast eIF2alpha kinases. Insect cells infected with wild-type, but not pk2-deleted, baculovirus exhibited reduced eIF2alpha phosphorylation and increased translational activity. The negative regulatory effect of human protein kinase RNA-regulated (PKR), an eIF2alpha kinase, on virus production was counteracted by PK2, indicating that baculoviruses have evolved a unique strategy for disrupting a host stress response. PK2 was found in complex with PKR and blocked kinase autophosphorylation in vivo, suggesting a mechanism of kinase inhibition mediated by interaction between truncated and intact kinase domains.

Identification of interprotein interactions between the subunits of eukaryotic initiation factors eIF2 and eIF2B

Modulation of protein/protein interaction is an important mechanism involved in regulation of translation initiation. Specifically, regulation of the interaction of eIF2 with the guanine nucleotide exchange factor, eIF2B, is a key mechanism for controlling translation under a variety of conditions. Phosphorylation of the alpha-subunit of eIF2 converts the protein into a competitive inhibitor of eIF2B by causing an increase in the binding affinity of eIF2B for eIF2. Consequently, it has been assumed that the alpha-subunit of eIF2 is directly involved in binding to eIF2B. In the present study, eIF2 was found to bind only to the delta- and epsilon-subunits of eIF2B, and eIF2B was shown to bind only to the beta-subunit of eIF2 by far-Western blot analysis. The binding site on eIF2beta for either the eIF2B holoprotein, or the isolated delta- or epsilon-subunits of eIF2B was shown to be located within approximately 70 amino acids of the C terminus of the protein. Phosphorylation of the alpha-subunit of eIF2 did not promote binding of eIF2B to the isolated subunit. However, it did cause an increase in the affinity of eIF2B for eIF2. Finally, phosphorylation by protein kinase A of the beta-subunit of eIF2 in the C-terminal portion of the protein increased the guanine nucleotide exchange activity of eIF2B, whereas phosphorylation by casein kinase II or protein kinase C was without effect.

Modulation of translation initiation in rat skeletal muscle and liver in response to food intake

Protein synthesis is altered in both skeletal muscle and liver in response to nutritional status with food deprivation being associated with an inhibition of mRNA translation. In the present study, the effect of food-intake on the initiation of mRNA translation was examined in rats fasted for 18-h and then refed a complete diet. Fasting and refeeding caused alterations in translation initiation in both skeletal muscle and liver that were not associated with any detectable changes in the activity of eIF2B or in the phosphorylation state of eIF2 alpha. Instead, alterations in initiation were associated with changes in the phosphorylation state of eIF4E and/or the association of eIF4E with eIF4G as well as the eIF4E binding protein, 4E-BP1. In muscle from fasted rats, the amount of eIF4E present in an inactive complex with 4E-BP1 was increased 5-fold compared to freely fed control animals. One hour after refeeding a complete diet, the amount of 4E-BP1 bound to eIF4E was reduced to freely fed control values. Reduced association of the two proteins was the result of increased phosphorylation of 4E-BP1. Refeeding a complete diet also stimulated the binding of eIF4E to eIF4G to form the active eIF4F complex. In liver, the amount of eIF4E associated with eIF4G, but not the amount of eIF4E associated with 4E-BP1, was altered by fasting and refeeding. Furthermore, in liver, but not in skeletal muscle, fasting and refeeding resulted in modulation of the phosphorylation state of eIF4E. Overall, the results suggest that protein synthesis may be differentially regulated in muscle and liver in response to fasting and refeeding. In muscle, protein synthesis is regulated through modulation of the binding of eIF4E to eIF4G and in liver through modulation of both phosphorylation of eIF4E as well as binding of eIF4E to eIF4G.

Brefeldin A inhibits protein synthesis through the phosphorylation of the alpha-subunit of eukaryotic initiation factor-2

Brefeldin A is a fungal metabolite which disrupts protein traffic through the Golgi apparatus and thereby inhibits protein secretion. Recently, it has been shown that Brefeldin A also causes a marked decrease in the rate of protein synthesis in cells in culture [1992, FEBS Lett. 314, 371-374]. We show here that treatment of rat GH3 pituitary cells with Brefeldin A leads to an inhibition of protein synthesis at the level of peptide-chain initiation through a mechanism involving the phosphorylation of the alpha-subunit of eukaryotic initiation factor-2 (eIF-2 alpha).

Supramolecular organization of the mammalian translation system

Although evidence suggests that the protein synthetic machinery is organized within cells, this point has been difficult to prove because any organization that might exist is lost upon preparation of the cell-free systems usually used to study translation in vitro. To examine this process under conditions more representative of the intact cell, we have developed an active protein-synthesizing system using Chinese hamster ovary (CHO) cells permeabilized with the plant glycoside saponin. This procedure renders cells permeable to trypan blue and exogenous tRNA, but there is little release of endogenous macromolecules. Protein synthesis in this system proceeds at the same rate as that in intact cells and is about 40-fold faster than that in a cell-free system prepared from the same cells. Active protein synthesis in this system requires the addition of only Mg2+, K+, and creatine phosphate, with a small further stimulation by ATP and an amino acid mixture; no exogenous macromolecules are necessary. The proteins synthesized in this system are indistinguishable from those made by the intact cell, and the channeling of aminoacyl-tRNA observed in vivo is maintained. Our data suggest that the permeabilized cell system retains the protein-synthesizing capabilities of the intact cell and presumably its internal structure as well. Studies with this system demonstrate that the protein-synthesizing apparatus is highly organized and that its macromolecular components are not freely diffusible in mammalian cells.

Purification of eukaryotic initiation factors eIF-2, eIF-2B and eIF-2 alpha kinase from bovine liver

Eukaryotic initiation factors 2 and 2B (eIF-2; eIF-2B) are components of the rate-limiting step in the initiation of eukaryotic protein synthesis and are involved in the regulation of this process. When the alpha-subunit of eIF-2 is phosphorylated by an eIF-2 alpha kinase, the phosphorylated eIF-2 alpha (eIF-2 alpha(P)) binds tightly to eIF-2B and prevents the recycling of eIF-2.GDP to eIF-2.GTP which is required for sustained initiation of protein synthesis. The minute quantities of these proteins which are present in rat liver and muscle cytosol along with hundreds of other proteins has hindered purification efforts, as well as structure:function and regulatory studies. Therefore, procedures were developed for the simultaneous purification of eIF-2, eIF-2B and eIF-2 alpha kinase from kilogram quantities of fresh bovine liver. Briefly, the 0-45% ammonium sulfate precipitate of the 200,000 x g supernatant was solubilized and chromatographed on DEAE-cellulose, heparin-agarose, Mono Q, Mono S, and Superose columns. The availability of purified quantities of these factors will be useful for investigations of molecular mechanisms of action and antibody production.

Translational initiation factor eIF-2 subcellular levels and phosphorylation status in the developing rat brain

We have quantified the levels of the alpha subunit of initiation factor 2 (eIF-2) in the postmicrosomal supernatant and the ribosomal salt wash fractions from suckling and adult rat brain. The levels of eIF-2 in the ribosomal salt wash decrease in adult with respect to that present in suckling rat brain, but the total amount remains fairly constant, and a very close parallelism exists between the eIF-2 associated with ribosomes and RNA levels in the microsomal fraction in the two age groups. The phosphorylation state of eIF-2 alpha, as determined by isoelectric focusing followed by protein immunoblotting, in the same subcellular fractions, did not reveal the presence of the phosphorylated form in any of the fractions studied. These results suggest that phosphorylation of the alpha subunit is not implied in the regulation of protein synthesis initiation during brain development, and some other component regulates both the number of active ribosomes and eIF-2 levels in microsomes.

Age-dependent decrease in the amount of eukaryotic initiation factor 2 in various rat tissues

Recent studies have suggested that the decline in protein synthesis that occurs in rat liver and brain during development and aging is associated with a decrease in the activity of eukaryotic initiation factor 2 (eIF-2). One way in which eIF-2 activity could be decreased in tissue extracts would be through a decrease in the activity of the GDP exchange factor, eIF-2B. In the present study, the activity of eIF-2B was measured in tissue extracts and was found to be less in older than in younger rats. Thus a decrease in eIF-2B activity could account for part of the decrease in protein synthesis that occurs during aging. Another way in which eIF-2 activity could be decreased would be through a decrease in amount of the protein. Therefore the amount of eIF-2 in various tissues was quantified by protein immunoblot analysis. We found that the amount of eIF-2 relative to total protein tended to fall with increasing age. Furthermore, eIF-2 content was directly proportional to the rate of protein synthesis in the tissues examined. Finally, slot-blot analysis of polyadenylated RNA revealed no significant change in the relative abundance of eIF-2 alpha mRNA with age. The last-mentioned experiments suggest that the synthesis of eIF-2 may be regulated through changes in the deficiency of translation of eIF-2 alpha mRNA rather than through changes in gene transcription.

Regulation of protein synthesis in Swiss 3T3 fibroblasts. Rapid activation of the guanine-nucleotide-exchange factor by insulin and growth factors

Insulin, whole serum, phorbol esters and epidermal growth factor each rapidly stimulate protein synthesis in serum-depleted Swiss 3T3 fibroblasts. The activation of protein synthesis by each of these agents is associated with stimulation of the activity of the guanine-nucleotide-exchange factor (GEF). This protein recycles the initiation factor eIF-2 by promoting exchange of GDP bound to eIF-2 for GTP. Activation of GEF is rapid, becoming maximal within 15 min. The degree of activation of GEF by these stimuli (to greater than 170% of control for insulin, serum or epidermal growth factor; 120% for phorbol dibutyrate) is more than enough to account for their effects on the overall rate of translation. Stimulation of protein synthesis and GEF activity occurs at low nanomolar insulin concentrations, indicating they are mediated through the insulin receptor. The best-characterized mechanism for regulating GEF activity is through changes in the phosphorylation of the smallest subunit of eIF-2 (eIF-2 alpha); however, none of the stimuli studied altered the level of phosphorylation of eIF-2 alpha in Swiss fibroblasts. It seems that direct regulation of GEF activity may be occurring here, and possible mechanisms for this are discussed.

Characterization of a cell-free protein synthesizing system from rat lung

1. A cell-free protein synthesizing system has been developed from a novel source, namely the rat lung. 2. The system translates endogenous mRNA at a linear rate for up to 10 min at approx 5% of the in vivo rate. 3. With the use of edeine and 7-methylguanosine-5'-triphosphate (m7GTP), specific blockers of peptide chain initiation, we have demonstrated that 40-60% of total amino acid incorporation is attributable to reinitiation on nascent polypeptide chains. 4. The lung cell-free system will be a valuable asset when investigating the mechanisms involved in the regulation of pulmonary protein synthesis.

Inhibition of microsomal calcium sequestration causes an impairment of initiation of protein synthesis in perfused rat liver

The present study examined the effect of 2,5-di-(tert-butyl)-hydroquinone (tBuHQ), an inhibitor of liver microsomal calcium sequestration, on initiation of protein synthesis in perfused rat liver. Perfusion of livers with a concentration of tBuHQ previously shown to completely inhibit microsomal calcium sequestration in isolated hepatocytes caused a 50% inhibition of protein synthesis. The inhibition was characterized by an increase in liver content of free ribosomal particles and a decrease in polysomes indicating that peptide-chain initiation was slowed relative to elongation. Furthermore, the inhibition was associated with a 7.5-fold increase in the proportion of the alpha-subunit of eukaryotic initiation factor 2 (eIF-2) present in the phosphorylated form and a reduction in the activity of eukaryotic initiation factor 2B (eIF-2B) to 37% of the control value. The results suggest that protein synthesis in rat liver is regulated directly by changes in intracellular calcium concentration through a mechanism involving modulation of the phosphorylation state of eIF-2 alpha.